Communication technology has progressed significantly in the past few years. Today, much information is carried over optical communications fiber. This technology, know as fiber optic technology allows the transport of information at data rates currently exceeding millions of bits of information per second. Part of the technology that enables this optical communication is the ability to direct light onto an optical fiber and switch that light appropriately. Typically, a number of optical fibers are combined into a fiber optic cable. When a fiber optic cable is carrying many individual signals over large distances, it is necessary to have the ability to switch those signals onto other fiber optic cables. A mesh of fiber optic cable infrastructure spans the world. At certain places in the mesh it is desirable to have the ability to switch the light signals from one fiber optic cable to another. A typical fiber optic cable may be comprised of a plurality of individual optical fibers bound together, for example, in a ribbon arrangement. A typical fiber optic ribbon cable may contain 32 individual optical fibers. Each optical fiber is capable of carrying one signal, or in the case of dense wave division multiplexing (DWDM), in which many signals may be multiplexed onto a single optical fiber through the use of multiple colors of light, each optical fiber may carry a plurality of light colors (wavelengths), each color carrying a single signal.
Optical switches capable of routing light from one direction to another have been known for some time. A new type of optical switch element, known as a non-blocking optical switch element, is disclosed in commonly assigned U.S. Pat. No. 5,699,462 to Fouquet et al., in which optical switch elements are located at an intersection of two optical waveguides. Depending on the state of a material within the optical switch element, light is either transmitted through the switch element continuing axially on the original waveguide, or reflected by the switch element onto a waveguide that intersects the original waveguide. The switch element is filled with a material that, while in a transmissive state, has an index of refraction substantially equal to that of the waveguide, thus allowing light in the waveguide to pass through the switch element. In this state the switch element is considered non-blocking. The state of the material within the switch element may be changed, through the operation of heaters or the like within the switch element, so as to cause a gas, or bubble, to be formed within the switch element. While present in the switch element the bubble causes a refractive index mismatch between the waveguide and the switch element, thus causing the light in the waveguide to be reflected onto the intersecting waveguide. This state is known as the reflective state. The operation of a preferred and many alternative embodiments of the non-blocking switch element is set forth in detail in commonly assigned U.S. Pat. No. 5,699,462 to Fouquet et al., which is hereby incorporated by reference.
When placed at an intersection of two waveguide segments, one of the above-mentioned optical switch elements forms an optical switch point, which may be used to switch signals on a plurality of optical fibers. The optical switch points may be further arranged so as to form a switching matrix. For example, when arranged in a 32.times.32 matrix, formed by 32 rows and 32 columns of optical switch points, a 32 fiber optic ribbon cable can be connected to 32 input lines and another 32 fiber optic ribbon cable can be connected to 32 output lines, the output lines intersecting the 32 input lines. Because a switch element is located at each optical switch point it is possible to switch any of the 32 input lines to any of the 32 output lines. In this manner, optical signals may be directed from one fiber optic cable onto another, resulting in a compact optical switch.
One drawback with the above-described optical switching matrix is that because there are exactly 32 inputs and 32 outputs for the two 32 fiber optic ribbon cables, if one optical switch point fails, there is no way in which to connect the respective input to the respective output corresponding to the failed optical switch point.
Therefore, it would be desirable if there were a way to provide fault tolerance and redundancy for the above-described optical switch matrix.